The present disclosure relates generally to ripple suppressors providing driving current, and more particularly to methods and ripple suppressors that could prevent a power transistor providing driving current from operating in a linear region.
In view of its high efficiency and low power consumption, light emitting diode (LED) has been widely adapted as a lighting source in daily life. In consideration of fabrication cost, the circuit for driving LED usually employs only one single stage of a power-factor-correction (PFC) power convertor. Standing alone, a PFC power converter is well known to have a considerably high output current ripple. One common solution to reduce this output current ripple is shunt at the output of a PFC power converter a capacitor of very large capacitance, which normally is an electrolyte capacitor. As known in the art, electrolyte capacitors are bulky in size and short in life span, and not welcome by modern LED products, which are usually requested to be compact and durable.
In order to avoid using an electrolyte capacitor, ripple suppressors are proposed to stabilize the current through LEDs. FIG. 1 demonstrates a LED system 10 including a bridge rectifier 12, a buck converter 14, a LED chain 16, and a ripple suppressor 18, configuration of which is shown therein. Buck converter 14 could provide PFC and constant average output current control as well. FIGS. 2A and 2B illustrate two ripple suppressors 18a and 18b in the art, each having a power NMOS transistor (Mna or Mnb). The gate voltage of the power NMOS transistor in each of FIGS. 2A and 2B results from low-passing the drain voltage of the power NMOS transistor. As the gate voltage should be stable, the drain-to-source current IDS of the power NMOS transistor, which flows through the path under the gate of the power NMOS transistor and is substantially equal to the current through the LED chain 16, is proximately constant, suppressing ripples of the drain-to-source current IDS.